Merge branch 'kf/gpg-sig-verification-doc' into maint

Documentation for "git merge --verify-signatures" has been updated
to clarify that the signature of only the commit at the tip is
verified.  Also the phrasing used for signature and key validity is
adjusted to align with that used by OpenPGP.

* kf/gpg-sig-verification-doc:
  Documentation: clarify signature verification

Documentation/merge-options.txt

@@ -89,8 +89,11 @@ option can be used to override --squash.
 
 --verify-signatures::
 --no-verify-signatures::
-	Verify that the commits being merged have good and trusted GPG signatures
+	Verify that the tip commit of the side branch being merged is
-	and abort the merge in case they do not.
+	signed with a valid key, i.e. a key that has a valid uid: in the
+	default trust model, this means the signing key has been signed by
+	a trusted key.  If the tip commit of the side branch is not signed
+	with a valid key, the merge is aborted.
 
 --summary::
 --no-summary::

Documentation/pretty-formats.txt

@@ -143,8 +143,8 @@ ifndef::git-rev-list[]
 - '%N': commit notes
 endif::git-rev-list[]
 - '%GG': raw verification message from GPG for a signed commit
-- '%G?': show "G" for a Good signature, "B" for a Bad signature, "U" for a good,
+- '%G?': show "G" for a good (valid) signature, "B" for a bad signature,
-  untrusted signature and "N" for no signature
+  "U" for a good signature with unknown validity and "N" for no signature
 - '%GS': show the name of the signer for a signed commit
 - '%GK': show the key used to sign a signed commit
 - '%gD': reflog selector, e.g., `refs/stash@{1}`